1. FIELD OF THE INVENTION
The invention relates to a process and apparatus for the regeneration of fluidized catalytic cracking catalyst.
2. DESCRIPTION OF RELATED ART
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor. The benefits of riser reactor FCC units are such that many older units have been revamped to take advantage of this advance in technology.
There have been many improvements in the design of FCC regenerators. The considerable evolution in the design of FCC units is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article.
Most new regenerators are of the high efficiency design, i.e., the spent catalyst, preferably with recycled regenerated catalyst, is charged to a fast fluidized bed coke combustor, and from their to a dilute phase transport riser. Coke is efficiently burned in the robustly fluidized coke combustor, while CO afterburning is promoted by the dilute phase conditions in the transport riser. Such regenerators are now the standard for new construction, and are shown in U.S. Pat. No. 4,820,404, Owen, U.S. Pat. No. 4,353,812, Lomas et al, and many others. These two patents are incorporated herein by reference.
Such modern regenerator designs, sometimes called a high efficiency regenerator, are preferred for all new construction. For the many FCC units built with low efficiency, i.e., bubbling dense bed regenerators, it has not been possible and/or economically justifiable to improve the efficiency of the bubbling bed regenerator.
Such bubbling bed regenerators are inherently inefficient because of the presence of large gas bubbles, poor catalyst circulation, and the stagnant regions. The bubbling bed regenerators usually have two to three times the catalyst inventory of more modern regenerators. The increased inventory, and longer catalyst residence time, make up for a lack of efficiency.
For such units, characterized by a single, bubbling dense bed regenerator, there has been no good way to achieve the benefits of high efficiency regeneration. Site constraints usually make replacement of a single bubbling bed regenerator with a high efficiency regenerator.
Site constraints also usually make modifications, such as those that would permit several stages of regeneration to be achieved in a single vessel, prohibitively expensive. Part of the difficulty is that usually some form of baffling or separation is needed to achieve multistage regeneration, i.e., the fluidized bed regions must be isolated, and the flue gas from each region must be isolated. Some means of recovering catalyst from flue gas is usually essential, because even in bubbling bed regenerators with relatively low superficial vapor velocities there is a tremendous amount of catalyst entrainment into the dilute phase. Multiple cyclones in parallel, with multiple stages of cyclones, i.e., in series, are usually needed to recover catalyst from flue gas. These cyclones are heavy, and difficult to support, and when multiple stages of catalyst regeneration are involved, and great swings in temperature must be accommodated in the regenerator, the problems of cyclone support, and thermal stress, multiply.
It was easy to isolate the fluidized bed regions--the catalyst acted like a liquid, and a simple solid baffle would effectively one fluidized region of catalyst from the other. Baffled regions, defining isolated fluidized beds sharing a common vapor region above, are common. U.S. Pat. No. 2,584,391 disclosed an apparatus with a baffled fluidized bed region which could be said to define multiple regions in a fluidized bed, but the vapor phases from each fluidized bed were mixed together and withdrawn from a single outlet. This was an improvement, it gave the option to achieve multiple stage regeneration, but added the constraint that the flue gas streams had to be compatible. If an attempt were made to operate the apparatus shown in U.S. Pat. No. 2,584,931 as a regenerator, with the inner stage in partial CO combustion mode, and the outer stage in complete CO burn mode, with an oxidizing atmosphere, the two flue gases would "afterburn" when mixed together in the dilute phase region above the dense beds. The lack of sufficient spent catalyst, to absorb the heat of combustion, would lead to extremely high temperatures in the flue gas line and in the cyclones, which could damage the unit.
Something better was needed, that would allow the beds to be isolated (this was easy) while keeping the flue gasses from the beds isolated (this was difficult).
I realized that some of the vices of these older regenerators were also virtues, and that two isolated fluidized beds could be accommodated in a single regenerator vessel. The large size of these vessels, large enough to hold 100's of tons of fluidized catalyst, and the relatively low gas velocities which were used to minimize entrainment catalyst entrainment with the flue gas, provided an ideal way to solve the problem. I discovered a way to effectively isolate the flue gas from each stage of regeneration, relying on laminar flow and the laws of physics to isolate the flue gas streams, rather than physical barriers. In a preferred embodiment I use a somewhat high superficial vapor velocity in one region to entrain additional catalyst into the dilute phase, and act as insurance or buffer, to minimize high temperature excursions when oxidizing and reducing flue gasses meet. In another embodiment I use unusually low superficial vapor velocities, and an unusual way of transferring catalyst from one region to another, to reduce pressure drop through the system and maximize coke burning capacity of the regenerator.